docs: def getone(self, key):
          """
          Get one value matching the key, raising a KeyError if multiple
          values were found.
          """
          v = self.getall(key)
          if not v:
              raise KeyError('Key not found: %r' % key)
          if len(v) > 1:
              raise KeyError('Multiple values match %r: %r' % (key, v))
          return v[0]